An integrated circuit (IC) connected to external terminals is vulnerable to electrostatic discharge (ESD) pulses from the operating environment and peripherals such as human bodies or machines. An ESD event may produce high current or high voltage pulses within a few nanoseconds, leading to device degradation or damage. In order to protect the vulnerable IC from ESD damaging pulses, an ESD protection circuit has to be connected to the external terminals of main IC elements.
Compound semiconductor devices have been widely used in the radio frequency (RF) circuits market. For example, compound semiconductor high electron mobility transistor (HEMT) switch have been widely applied in 3G cell phone applications in recent year for their high performance in RF range. However, the lack of good ESD protection devices has become a major drawback in HEMT switch applications. Conventionally, ESD protection circuits are made of series-connected diodes. A single enhancement-mode FET (E-FET) with the gate connected to the source with a resistor can act as an E-FET diode with different turn-on voltage for forward and reverse bias voltage Von—forward and Von—reverse, as shown in FIG. 7A. The Von—forward is small and is equal to the pinch-off voltage of the E-FET, and the Von—reverse is large and is determined by the voltage drop across the resistor due to the reverse gate leakage current. The Von—reverse can be adjusted to some extent by using a resistor with different resistance. An E-FET with its gate connected to the source by a resistor can thus be used as an ESD protection device. As shown in FIG. 7B, the positive and negative turn-on voltages (Von—j, and Von—n) can be multiplied by series connection of more than two E-FET diodes in the same direction. If two diodes are connected in opposite directions, both the positive and negative turn-on voltages (Von—j, and Bo—n) are determined by the reverse turn-on voltage of the single diode as shown in FIG. 7C. By applying series connected E-FET diodes for each direction, both of the overall positive and negative turn-on voltage can be adjusted (FIG. 7D).
Consider a circuit with an ESD protection device using E-FET diodes, as shown in FIG. 8A. For the ESD protection device to remain turned off during the circuit operation, Von—p and Von—N must satisfy the equations below at any instance:Von—N<Vb−Va<Von—P,where Va and Vb are voltages at the two terminals of the circuit under operation. For the case shown in FIG. 8B, Von—p and Von—N must satisfy the equations below at any instance:Von—N<Va<Von—P.
To meet the above conditions, the number of series connected E-FET diodes needed could be large. With increasing number of diodes connected in series in the same direction and/or the opposite direction, the area occupied by the whole ESD protection device becomes large, which results in the increase in the total chip size.